As geometries continue to shrink, manufacturers have increasingly turned to optical techniques to perform non-destructive inspection and analysis of semiconductor wafers. The basis for these techniques is the notion that a subject may be examined by analyzing the reflected energy that results when an optical beam is directed at a subject. This type of inspection and analysis is known as optical metrology and is performed using a range of different optical techniques.
One widely used type of optical metrology system, as shown in FIG. 1, includes a pump laser. The pump laser is switched on and off to create an intensity-modulated pump beam. The pump beam is projected against the surface of a subject causing localized heating of the subject. As the pump laser is modulated, the localized heating (and subsequent cooling) creates a train of thermal and plasma waves within the subject. These waves reflect and scatter off various features and interact with various regions within the sample in a way that alters the flow of heat and/or plasma from the pump beam spot.
The presence of the thermal and plasma waves has a direct effect on the surface reflectivity of the sample. Features and regions below the sample surface that alter the passage of the thermal and plasma waves will therefore alter the optical reflective patterns at the surface of the sample. By monitoring the changes in reflectivity of the sample at the surface, information about characteristics below the surface can be investigated.
To monitor the surface changes, a probe beam is directed at a portion of the subject that is illuminated by the pump laser. A photodetector records the intensity of the reflected probe beam. The output signal from the photodetector is filtered to isolate the changes that are synchronous with the pump beam modulation. For most implementations, this is performed using a heterodyne or lock-in detector (See U.S. Pat. No. 5,978,074 and in particular FIG. 2 for a discussion of such a lock-in amplifier/detector). Devices of this type typically generate separate “in-phase” (I) and “quadrature” (Q) outputs. These outputs are then used to calculate amplitude and phase of the modulated signal using the following equations:Amplitude=√{square root over (I2+Q2)}  (1)Phase=arctan(Q/I)  (2)
The amplitude and phase values are used to deduce physical characteristics of the sample. In most cases, this is done by measuring amplitude values (amplitude is used more commonly than phase) for one or more specially prepared calibration samples, each of which has known physical characteristics. The empirically derived values are used to associate known physical characteristics with corresponding amplitude values. Amplitude values obtained for test subjects can then be analyzed by comparison to the amplitude values obtained for the calibration samples.
Systems of the type shown in FIG. 1 (i.e., those using external means to induce thermal or plasma waves in the subject under study) are generally referred to as PMR (photomodulated reflectance) type systems. PMR-type systems are used to study a range of attributes, including material composition and layer thickness. PMR-type systems and their associated uses are described in more detail in U.S. patent Ser. Nos.: U.S. Pat. Nos. 4,634,290; 4,646,088; 4,679,946; 4,854,710; 5,854,719; 5,978,074; 5,074,669; and 6,452,685. Each of these patents is incorporate in this document by reference.
Another important use of PMR-type systems is measurement and analysis of the dopants added to semiconductor wafers. Dopants are ions that are implanted to semiconductors during a process known as ion implantation. The duration of the ion implantation process (i.e., total exposure of the semiconductor wafer) controls the resulting dopant concentration. The ion energy used during the implantation process controls the depth of implant. Both concentration and depth are critical factors that determine the overall effectiveness of the ion implantation process.
PMR-type systems are typically used to inspect wafers at the completion of the ion implantation process. The ion implantation damages the crystal lattice as incoming ions come to rest. This damage is typically proportional to the concentration and depth of ions within the crystal lattice. This makes measurement of damage an effective surrogate for direct measurement of dopant concentration and depth. PMR-type systems have proven to be adept at measuring damage and have been widely used for post implantation evaluation.
As shown in FIG. 2, the relationship between dopant concentration and amplitude measurements (i.e., as defined by Equation (1)) is monotonic for low dopant concentrations. As dopant concentrations increase (e.g., greater than 1E14 for As+ or P+ ions or greater than 1E15 for B+ ions) the monotonic relationship breaks down. In fact, at high concentrations, the amplitude measurements are no longer well behaved and as a result cannot be used to accurately derive corresponding dopant concentrations. In FIG. 1, this is illustrated by the points A, B and C all having the same the same amplitude measurement, even though each point represents a different dopant concentration. The same sort of breakdown occurs as the type of implanted ions becomes heavier (e.g., As+ or P+ ions). In both cases, this is attributable to the appearance of a Si amorphous layer resulting in optical interference effects. Although not shown in FIG. 2, phase information becomes flat or insensitive to changes in concentration at high dopant concentrations or where heavy ions are implanted.
One approach for dealing with the problem of monitoring samples with high dopant concentrations is to measure the DC reflectivity of both the pump and probe beams in addition to the modulated optical reflectivity signal carried on the probe beam. Using the DC reflectivity data at two wavelengths, some ambiguities in the measurement can often be resolved. The details of this approach are described in U.S. Pat. No. 5,074,669 (incorporated in this document by reference).
In general, PMR-type systems of the type described above have proven to be effective methods for testing and characterizing semiconductor devices. Their ability to function in a non-contact, non-destructive fashion, combined with their high-accuracy and repeatability have ensured their widespread use as part of semiconductor manufacturing. Still, there is an obvious need for methods to provide this type of measurement capability for high dopant concentrations and ion implantation of relatively heavy ions.